Knife gate valves have traditionally been used to control the flow of heavy slurries transported through tubular conduits. The knife gate has a blade portion with a beveled edge to cut through the material suspended in the slurry as the knife gate is lowered to seal off the flow through the valve. The knife gate typically closes against the metal valve body, and the slurry assists in sealing the valve.
A problem associated with using a knife gate to control the flow of slurries of light viscosities is that the material accumulating on the surface of the valve seat prevents a tight seal. Therefore, knife gate valves controlling the flow of light viscosity slurries typically employ a resilient gate seat liner that is squeezed between the knife gate and the metal valve body when the valve is in the closed position.
When an adhesive slurry (e.g., paper pulp) is being handled, however, the slurry material that accumulates on the gate seat liner tends to cause the Wife gate to adhere to and pull out the seat liner when the knife gate is retracted to open the valve. The flowing slurry may then carry the seat liner downstream in the conduit system. Although lubricating the gate-contacting surface of the valve seat liner can reduce surface friction and thereby prevent adhesion of the knife gate to the seat, the lubricating fluid may be quickly carried away by moving slurry.
To avoid this problem, a valve has been introduced that comprises two, essentially flat, valve body members bolted together about a resilient gate seat. Each bolt runs through a bolt hole in the resilient gate seat to positively retain it. Two principal problems occur with this design. The first problem arises during operation. The essentially flat valve body members do not constrain the sealing member in the radial dimension (defined relative to the center of the flow opening). As a result, when the valve is closed, the knife gate pushes the resilient gate seat radially outwardly. Although the fasteners running through the sealing member constrain this radial movement, enough deformation is possible between each pair of fasteners that the effectiveness of the seal is reduced.
The second problem arises during manufacturing. If one bolt is fastened more tightly than another, the gate seat will be compressed more in one spot than in another. This reduces the effectiveness of the seal. It is even possible that pressurized slurry could leak around one or more loosely tightened fasteners. Tightening all the bolts equally, which is necessary to ensure a good seal, increases the expense and complicates the manufacturing process.